The present invention relates to a new and improved method of producing constituent materials for gas sensors essentially consisting of at least one metal oxide with at least one catalyst additive.
Such materials and methods for their preparation are known. It has already been known for a long time that n-type semiconductors consisting of metal oxides adsorb reducing or oxidizing gases at their surface as a result of which a change in the electrical resistance occurs. Certain catalysts have been added to the metal oxides in order to increase the sensitivity with respect to reducing or oxidizing gases. Thus, a method of producing a material for gas sensors is known, for example, from German Pat. No. 2,428,488, published Jan. 9, 1975, which material contains tin dioxide as a base material and platinum as a catalyst. Furthermore, palladium is considered therefor as a metal catalyst and copper (II) oxide or nickel (II) oxide are considered as metal oxide catalysts therefor.
According to the known methods the base material is solely doped by impregnating the metal oxide powder with a solution of a precious metal salt like, for example, hydrogen hexachloroplatinate or disodium tetrachloropalladiate or a solution of a copper or nickel salt like, for example, copper chloride or nickel nitrate and by subsequent drying and heating. During the drying operation the metal salt is deposited in the form of small crystals and the metal salt decomposes during heating as a result of which the precious metal platinum or palladium remains in metallic form while the copper or nickel remains in the form of the metal oxide. The metal or metal oxide grains generally have an average diameter in the range of 10 to 500 nm. The local distribution of the grains, however, is not homogeneous since during the drying of the solution high local concentration differences occur at the metal oxide powder.
Gas sensors which are produced from such metal oxide powder, therefore, have highly variable sensitivities which differ from one sensor to the other. Furthermore, this method has the disadvantage that the anions of the metal salt or metal complex like, for example, the chloride or nitrate ions are bonded to vacancies in the metal oxide and so uncontrollably affect the conductivity of the metal oxide powder. This, in turn, results in variations in the sensitivity of the individual sensors.
In order to at least partially diminish the last mentioned effect the metal oxides were heated to temperatures of at least 600.degree. to 1000.degree. C. during the preparation in order to expel the aforementioned anions. At such high temperatures, however, the metal oxides sinter with a large volume reduction and then only have a small porosity. The general sensitivity of the gas sensor is thereby highly reduced.
In a method of preparing gas sensors as known, for example, from U.S. Pat. No. 4,362,765, granted Dec. 7, 1982, a mixture of a metal with its oxide is evaporated at reduced pressure in an oxygen-containing atmosphere, whereby there is obtained a layer of particles having an average diameter in range of 1 to several 10 nm on an insulating material which is provided with electrodes. The material for gas sensors obtained according to this procedure, however, has only low mechanical strength. When, for example, palladium is intended to be added as a catalyst to the oxide material which, for example, consists of tin dioxide, the catalyst can not be homogeneously distributed therein but has to be deposited thereon in a number of separate layers.
The photochemical deposition of metals on metal oxide powders is described in U.S. Pat. No. 4,264,421, granted Apr. 28, 1981, and is used for the preparation of catalyst powders. Such method requires acetate ions and a reaction temperature of up to 60.degree. C. for an effective metal deposition.
The method according to the invention which is further described hereinafter basically differs from the method as known from the aforementioned U.S. Pat. No. 4,264,421:
Instead of a metal oxide powder which is suspended in a solution, there is used according to the inventive method a colloidal solution of metal oxides, metal oxide hydrates or metal hydroxides which is mixed with a metal salt solution and then irradiated.
The photochemical reaction is conducted at room temperature or even below. It is thus prevented that the irradiated colloidal solution coagulates. The acetate ions required for an efficient photoreaction according to U.S. Pat. No. 4,264,421 cause the colloidal solutions to coagulate.
In order to accelerate the photochemical reaction, therefore, no acetate ions are added in the method according to the invention as described further hereinafter, but there are added alcohols, ketones or other reductants which do not generate ionic products during oxidation or do not change the hydrogen ion concentration in the solution to such an extent that the irradiated colloidal solution would coagulate.
Contrary to this U.S. Pat. No. 4,264,421 there is not present a suspension after the irradiation of the mixture with ultraviolet or visible light in the method according to the invention but a true colloidal solution. This solution can be processed via a solid in order to obtain large pieces of metal oxide gels. When starting from a metal oxide suspension as described in U.S. Pat. No. 4,264,421, pieces of gel cannot be obtained under any circumstances; however, gel pieces are required for the preparation of the present extremely sensitive gas sensors. The solution as mentioned hereinbefore also can be lyophilized which yields the solid in the form of a fine powder. This powder consists of colloidal particles of the irradiated solution which have a much smaller diameter than the metal oxide grains utilized according to U.S. Pat. No. 4,264,421. Only when the herein described extremely small metal oxide particles are used, can there be ultimately produced the herein described gas sensors which are unique in their quality.
Further and again contrary to U.S. Pat. No. 4,264,421 the mixtures do not turn colorless during irradiation with light, but generally turn into a deep black color. The reason therefor is that the deposited metal is extremely finely distributed and is extremely finely deposited upon the colloidal metal oxide particles.